1. Field of the Invention
The present invention generally relates to a single-phase induction motor and, more particularly, to a voltage-controlled electronic relay for starting a single-phase induction motor.
2. Description of the Related Art
Conventionally, a single-phase induction motor that is operated with single-phase AC power includes an operation coil and a starting coil. The starting coil becomes conductive only at the moment when the motor starts to provide a starting rotary power to the motor and is maintained in OFF state when the motor is in its normal operation state after started. A device for turning on/off the starting coil of the single-phase induction motor is called a centrifugal switch or starting relay. A voltage-controlled electronic relay employs the characteristic that the voltage induced across both ends of the starting coil increases as the speed of revolution of the motor is raised. That is, the voltage-controlled relay provides power to the starting coil at the initial stage where the induction motor is supplied with power, and then detects the voltage induced across the starting coil to cut off the power applied to the starting coil when the induced voltage becomes higher than a predetermined level (when the motor reaches its normal operation state). In case that a heavy load is applied to the motor during its normal operation so that the motor becomes a constraint state, the voltage induced across the starting coil is lowered. The relay senses this induced voltage and provides power to the starting coil when the voltage becomes lower than a predetermined level to start the motor again. Here, the induced voltage turning off the relay is relatively high and the induced voltage turning on the relay again is relatively low, the difference between the two voltages being called xe2x80x98hysteresis widthxe2x80x99.
A conventional voltage-controlled electronic relay for starting the single-phase induction motor includes the voltage-controlled electronic relay disclosed in Korean Pat. No. 91-2458 applied by the applicant. This voltage-controlled relay includes a single-phase induction motor 110, and an electronic relay circuit 120 for turning on/off the starting coil of the motor, as shown in FIG. 1.
Referring to FIG. 1, the single-phase induction motor 110 has operation coils W1 and W2 and the starting coil W3. The operation coils W1 and W2 are connected such that they directly receive commercial AC power (AC 110V) through power input ports L1 and L2 but the starting coil W3 accepts the power via a starting capacitor SC and the electronic relay 120.
The electronic relay 120 that is a switch for applying the power to the starting coil W3 through the starting capacitor SC is constructed of a triac 121 and a control circuit for triggering the gate of the triac 121. The control circuit includes a power supply unit 122 for supplying power to circuit elements of the relay, a control signal generator 123 for sensing the voltage across the starting coil W3 to generate an ON/OFF control signal, and a triggering unit 124 for triggering the gate of the triac 121 according to the output of the control signal generator 123.
The power supply unit 122 consists of a diode D2 for rectifying the AC power applied through connection ports T1 and T2, a filter capacitor C4 for filtering the output of the diode D2, distribution resistors R7 and R8, a zener diode ZD and a filter capacitor C2, to supply power Vcc to NAND gates M1, M2, M3 and M4.
The control signal generator 123 is constructed of a diode D1 and distribution resistors R1 and R2 for sensing the voltage across the starting coil W3, a resistor R3 for controlling the hysteresis width and two NAND gates M1 and M2, to sense the voltage induced across the starting coil W3 to generate the control signal for turning on/off the triac 121. The triggering unit 124 includes NAND gates M3 and M4 for creating oscillation according to the control signal and a transistor TR for interrupting a pick-up coil PC, the pick-up coil PC triggering the gate of the triac 121. Here, the output of the NAND gate M2 is positively fed back to the NAND gate M1 through the resistor R4 to widen the hysteresis width and the capacitor C3 and resistor R5 negatively feeds back the output of the NAND gate M4 to create oscillation. In FIG. 1, reference symbols R4 and R9 designate current-limiting resistors and C1 represents a filter capacitor.
When the AC power is applied to the induction motor 110 having the aforementioned configuration, the power Vcc is supplied to the circuit elements through the power supply unit 122 to operate the electronic relay 120. The voltage induced across the starting coil W3 is applied to the NAND gate M1 through the diode D1 connected to the connection port T3, the distribution resistors R1 and R2 and the hysteresis width controlling resistor R3. At this time, a low-level signal is inputted to the NAND gate M1 at the initial stage because the voltage induced across the starting coil W3 is low. The NAND gate M1 inverts this low input signal into a high signal to transmit it to the NAND gate M3. By doing so, the oscillation circuit configured of the NAND gates M3 and M4 oscillate. The oscillating signal of the NANG gate M4 turns on/off a transistor TR to interrupt the primary coil of the pick-up coil PC so that a signal voltage capable of triggering the gate of the triac 121 is induced to the secondary coil of the pick-up coil PC, to thereby turn on the triac 121. When the triac 121 is turned on, the starting coil W3 is provided with the AC power through the triac 121 and the starting capacitor SC to start the single-phase induction motor 110.
When the speed of revolution of the motor 110 increases according to the starting operation thereof, the voltage induced across the starting coil W2 is also raised gradually. If this induced voltage reaches a predetermined voltage set by the hysteresis width controlling resistor R3, the level of the signal applied to the NAND gate M1 becomes high so that the NAND gate M1 outputs a low-level signal. This interrupts the oscillation operation of the NAND gates M3 and M4 and triggering of the gate of the triac 121 through the pick-up coil, to thereby turn off the triac 121.
When the triac 121 is turned off, the AC power applied to the starting coil w3 through the starting capacitor SC is cut off, and the induction motor 110 is operated only by the operation coils W1 and W2.
FIG. 2 is a circuit diagram of another voltage-controlled electronic relay for starting the single-phase induction motor. The operation of the circuit of FIG. 2 is similar to that of the circuit shown in FIG. 1, excepting that a resistor R10 and a capacitor C6 are connected in parallel between the gate and cathode of the triac to be connected to the secondary coil of the pick-up coil PC and the output signal of the NAND gate M4 is transmitted to the primary coil grounded through a capacitor C5, to control the triac.
The conventional voltage-controlled electronic relay described above can control the positive feedback characteristic of the NAND gate and the intensity of input signal to widen the hysteresis width up to 75V that is above half the power supply voltage. Accordingly, the starting device can be stably operated even in a power equipment area where voltage variation is severe. In addition, there is no generation of arc and the device can be installed in any place. However, the conventional relay has problems that the voltage applied to the NAND gates M1 to M4 is not stable and the triac may be damaged due to impulse noise.
It is, therefore, an object of the present invention to provide a voltage-controlled electronic starting relay for a single-phase induction motor, which has a spark killer for removing impulse noise, connected in parallel with a triac, to protect the triac from the impulse noise and provide stabilized voltage to circuit elements of the relay.
To accomplish the object of the present invention, there is provided a voltage-controlled electronic relay for starting a single-phase induction motor, comprising; a power supply unit, configured of a bridge diode, for supplying power to circuit elements of the starting relay when AC power of the induction motor is turned on; a switch for applying the AC power to a starting coil of the induction motor or cutting off the AC power; a sensing element for sensing a voltage induced to the starting coil; a hysteresis unit for outputting an ON control signal at the initial starting stage, generating an OFF control signal for turning off the switch when the induced voltage sensed by the sensing element reaches a predetermined OFF reference voltage, and generating the ON control signal for turning on the switch again when the induced voltage becomes lower than a predetermined ON reference voltage during a normal operation period; and a triggering unit for turning on the switch according to the ON control signal of the hysteresis unit and turning off the switch according to the OFF control signal.